The fluoropolymers (FP) are known for their high chemical resistance. Also amongst the polymers, they have high temperature and wear resistance. The most well-known fluoropolymer is polytetrafluoroethylene (PTFE) having the formula —(F2C—CF2)n—. Fluoropolymers are widely used in harsh conditions. They are used in anticorrosive seals, in chemical pipes and valves and in bearings. Another application range is in antiadhesives. Also fluoropolymers are used in high-temperature electronic parts.
Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is a branch of mechanical engineering. Materials that are used in tribology have to provide low friction and wear rate and conduct frictional heat. Also they have to carry large normal stresses and even carry electrons in some applications. The main drawbacks of polytetrafluoroethylene are its low wear resistance and thermal conductivity, although it possesses very low friction coefficient. The low friction coefficient of the PTFE is due to its molecular structure. It has un-branched chain-like molecular structure. The chains are linked only with weak Van der Waals forces. Thus the chains slide easily past each other. This results in transfer film formation. However, as the formation and removal cycle is repeated continuously, the wear rate increases and this procedure results in a high wear rate typical to PTFE.
Other common fluoropolymers include poly(vinylidene fluoride) (PVDF), hexafluoropropylene (HFP), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), fluorinated polyimide (FPI) and others.
Nanodiamond (ND) also referred to as ultrananocrystalline diamond or ultradispersed diamond (UDD) is a unique nanomaterial which can be easily produced in hundreds of kilograms by detonation synthesis. There are also other alternative synthesis methods for producing nanodiamonds.
Nanodiamonds can be produced by detonation process of trinitrotoluene (TNT) and hexogen (RDX, royal demolition explosive). The detonation is executed in steel chamber. The nanodiamonds are formed in the high pressure and temperature of the explosion. The chamber is cooled fast after the explosion. The detonation results in diamond blend (DB), which comprises nanodiamonds, amorphous carbon, graphite-like structures and metallic impurities. The content of nanodiamonds in the diamond blend is typically between 30 and 75% by weight. The pure nanodiamonds are extracted by chemical purification process. The diameter of the commercial nanodiamond is about 4 to 5 nm. Still, the nanodiamonds have the tendency to agglomerate and the diameter of the agglomerates can be several micrometers.
Nanodiamonds or nanodiamond compositions can be used, for example, in oils, lubricants, abrasives, coatings, cleaning agents etc.
It has been suggested that low loadings of nanoparticles have an effect on PTFE wear if several factors coincide. First, nanoparticles modify the crystalline morphology of the polymer. Traditional fillers can only reinforce the polymer mechanically. Reinforcement of traditional fillers can comprise supporting loads, initiating crazes and interrupting crack propagation. However, nanoparticles are the size of the polymer lamellae. Thus, the crystallinity of the polymer can alter with the addition thus changing several other physical properties. Second, the addition of nanoparticles can reduce the abrasion. The abrasiveness is lower than with traditional fiber additives or microparticles. Nanofillers polish the rough surfaces while removing little amount of material and thus preparing the surfaces for transfer films. Third, the fluorinated polymers filled with nano-additives can form stable transfer films. These films form when the subsurface damage is low and there are no large abrasive particles to disturb the formation. Films are well adhered, they protect the polymer from the counterface and also the counterface is protected from the abrasives. The film is an interface that has low shear strength. The decomposition of the filler can generate reaction products between the filler and the PTFE, which products can improve the bonding of a transfer film.
According to the literature PTFE composite coatings loaded with various substances have been studied. Such substances include silica, metallic nanoparticles, nanodiamond, nano-attapulgite, lanthanum oxide, titanium oxide, Kevlat fabric, grapheme, multiwalled carbon nanotubes (MWCNT) and single wall carbon nanotubes (SWCNT).
Lee J-Y. et al., Tribological behavior of PTFE film with nanodiamond. Surface & Coatings Technology. 188-189 (2004), pages 534-538, studied the behavior of PTFE films with a nanodiamond addition. Nanodiamond was supposed to enhance the wear resistance while maintaining low coefficient of friction (COF) of PTFE. This was due to the nanoscaled size, good mechanical properties and thermal conductivity. Nanodiamond-PTFE composite slurry was mixed from nanodiamonds dispersed in distilled water and PTFE aqueous suspension. The slurry was sprayed on aluminum substrate. The nanodiamond content in the composite was in the range of 0-4 wt. %. A heat treatment was performed for the coating. The film thickness was 30 μm. The tribological properties of the film were examined with ball-on-plate test apparatus. The wear was evaluated by measuring the width of the worn track.
The optimum wear properties were achieved at 2 wt. % nanodiamond content. The wear increased both with decreasing nanodiamond content and also with higher contents. COF was reduced with nanodiamond addition, from 0.21 to 0.16 at room temperature and from 0.12 to 0.08 at 150° C. Also the COF reached its optimum value at 2 wt. % nanodiamond content. The increased wear with higher nanodiamond contents was linked with the increasing agglomeration rate of nanodiamonds. Also COF increased slightly with higher contents, which could be due to the agglomeration. The tribological behavior of the composite was dominated by creation of thin transfer layer. Transfer layer, which reduces friction and wear, was formed by PTFE and nanodiamonds that are torn off the matrix. It was proposed that nanodiamonds roll within the film thus contributing to lowering the frictional force in the interface.
Lee J-Y. et al., Tribological behavior of PTFE nanocomposite films reinforced with carbon nanoparticles, Composites Part B: Engineering, 38:7(2007), pages 810-816, examined the tribological properties of PTFE-carbon nanoparticle-nanocomposites. PTFE coatings were reinforced with onion-like-carbon (OLC). OLC was produced by annealing nanodiamonds in temperature range of 1000-1900° C. Nanodiamonds were observed to retain their structure until 1000° C. They started to convert to graphitic sheets when the temperature reached 1300° C. The annealed particles were dispersed by attritional milling. OLC-PTFE slurry was obtained by mixing the aqueous dispersions of them, only that OLC dispersion contained an anionic surfactant to improve the dispersion rate. The slurry was coated on aluminum substrate. Coating thickness was 10 μm. The tribological properties were investigated with ball-on-plate-test.
The nanodiamonds and carbon onions are themselves solid lubricants and thus they didn't increase the friction coefficient, which occurs normally when PTFE is filled. The lowest friction coefficient was achieved with unannealed nanodiamonds, although the effect of annealing to the friction coefficient was negligible. All the measured values of COF were at the same level as pure PTFE. The nanodiamond (before annealing) already reduces the wear coefficient. Moreover, the lowest wear coefficient was achieved with the carbon onions treated in 1000° C. The heat treatment reduces the bonding strength between the nanodiamond particles. Especially at 1000° C. the effect of the reduced bonding was observed, the smallest particle size was achieved with this treatment.
The dispersion of the nanoparticles is affected by the bonding strength and surface conditions. The wear properties relate to size effects and the surface properties. Thus, the best wear resistance was achieved with particles that were heat treated at 1000° C. as fillers. This was because they had smallest particle size and could be well dispersed in the PTFE matrix. The damage amount at worn surfaces was reduced in PTFE composites with the addition of untreated nanodiamonds and the ones treated at 1000° C.
Lai S-Q. et al., The friction and wear properties of polytetrafluoroethylene filled with ultrafine diamond. Wear, 260:4-5 (2006), pages 462-468, studied the tribological properties of polytetrafluoroethylene-nanodiamond-composites. The nanodiamonds, which were used as PTFE fillers, were purified. The purified nanodiamonds had an averaged particle size of 10 nm, although they were agglomerated to larger clusters. The PTFE was in aqueous dispersion, in which the nanodiamonds were mechanically mixed. The composite was molded into blocks that were sintered by heating. The nanodiamond loading was 0-10 wt. %. The nanodiamonds in the PTFE matrix were poorly dispersed and were aggregated into clusters with diameters from several hundred nanometers to several micrometers. Wear test was performed to the composite with a block-on-ring experimental arrangement.
According to Lai S-Q. et al., the lowest friction coefficient (0.18) was achieved at 0.5 wt. % nanodiamond content. The wear decreased sharply as the nanodiamond content increased but turned to decrease slightly after the nanodiamond content exceeded 3 wt. %. When the worn surfaces were examined, it was discovered that the nanodiamond particles had congregated on worn surface. Under wear, nanodiamonds have a load-carrying capacity. They can also roll and slide under friction process. Thus, it was concluded that the macromolecular sliding friction of pure PTFE changed to mixture of sliding and rolling friction in the composites. The transfer film was formed in the interface and the steel counterface was not abraded by the composite. The debris of the composite was smaller than that of pure PTFE, which indicated that nanodiamond addition inhibited the formation of larger debris particles. Thus the wear was reduced.
In view of the above, there exists a demand for a fluoropolymer composite coating having improved properties. The aim of the invention is to provide a fluoropolymer composite coating having improved tribological properties.